If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in St. Helens: The hottest months are usually August and July, with an average temperature of 17°C, while the coldest months are February and December, with an average of 4°C. The rainiest months in St. Helens are December, January, November and March, with each month seeing an average of 190 mm of rainfall.